Backward wave tube



Feb. 18, 1958 J. R. PIERCE- ETAL 2,324,256

BACKWARD WAVE TUBE Filed Aug. 24, 1954 2 Sheets-Sheet 1 I ll J. R.PIERCE INVEIVZORS. WH VOCOM YM M' ATTORNEY Feb. 18, 1958 J. R. PIERCEETAL 2,824,256

BACKWARD WAVE TUBE Filed Aug. 24, 1954 2 Sheets-Sheet 2 OUTPUT FIG. 3 f20 4/ u aa $1 FORWARD ATTORNEY BACKWARD WAVE TUBE Application August 24,1954, SerialNo. 451,731 j 7 Claims. Cl. 315-35 This invention relates todevices which employ the nitecl States PatentOfiFice interaction betweena traveling electromagnetic wave and d an electron beam over a plurality.of operating wave-. lenths. Such devices are now generally termedtraveling wave tubes.

An important class of traveling wave tubes is one in which the electronbeam interacts with an electromagnetic wave which is propagating alongan interaction circuit in a direction opposite to that of the electronflow past the circuit. Such operation is termed backward wave operation.In backward wave operation the electromagnetic wave is made to propagatealong an interaction circuit of a kind which sets up electric fieldcomponents which have a phase velocity in a direction opposite to thatof energy propagation, and the velocity of the electron beam is madesubstantially equal to that. of a selectedoneof such components forinteraction therewith with aconsequent amplification of the propagatingwave. 'In a 'copending application SerialNo. 422,613,,filed April 12;1954, by A. Karp and W. C..Yocom there. is described a traveling'wavetube which incorporates an' interaction circuit Well adapted forbackward wave oper- 2,824,256 Patented Feb. 18, .1958

A related object is to secure such improvement by a more efficientinteraction circuit.

T 0 such ends, a feature of the invention is an interaction circuitwhich comprises a plurality of wire-like elements extending transverselyacross a wave guidein an axial linear array, characterized in that eachof the wire elements has a length which is longer than one half thecharacteristic cutoff wavelength of the wave guide. This end isadvantageously achieved by modifying the geometry of the guide to reduceits characteristic cutoff wavelength. To this end, the wave guide isridged at regions opposite the ends of the wires. This may be viewed,alternatively, as a reduction in the capacitance at the center of thewires relative to the capacitance at their ends or as the addition of aninductance at the two ends ofthe wires.

In a preferred embodiment of the invention, the wave guide is ridgedopposite the ends of the wires both below and above thelwires. Thisresults in an interaction circuit when comprises a wave guide having asubstantially -cruciform interior in which a plurality of wire elementsl extend in a linear array between a pair of side walls.

ation, particularly v because of the ease with which'it can I befabricatedainra form suitable forquse at millimeter wavelengths. Thisinteraction circuit comprises a linear array of straight wirelikeelements extending between the narrow side walls'ofa hollow wave guidewhich is provided with acentral ridge extending axially therealong inthe direction of the linear array. In this circuit,- the central ridgein the wave guide acts to increase the cutoff wavelength ofthe guidewhereby thelength of each wire element is shorter than the one half thecutofi wavelength of the guide. Such a circuit when propagating a signalwhose wavelength is'slightly longer than twice the length of the wiresis found to establish relatively strong axial electric field componentswhich have a phase velocity in the direction opposite to that of wavepropagation and so are conducive to backward wave operation. If theaxial electric field associated with the propagation of the wave alongthe circuit is developed as a sum of a Fourier series of which eachcomponent is traveling with a different phase velocity, the phasevelocity of the backward traveling component with which interaction ismost profitably'made is found to involve a (21r-0) term where 0, is thephase difference between adjacent wires with progress in the directionof the electron flow. This component is generally termed the firstnegative spatial harmonic.- i 7 Operation with such negative spatialharmonic components is found to possesscertain limitations. In general,it would be preferable to'interact with the fundamental component of aFourier series representation of thewave Q Each circuit: of this kind isfound to have associated with the propagation therealong of a wave inone direction an axial electric field component with a phase velocity inthe opposite direction which in a Fourier series representation of theaxial electric field of the wave involves simply 0 where 0 is, asbefore, the phase shift between adjacent wires with progress in thedirection of electron flow. Such a component represents the fundamentalof the Fourier series representation of the wave, and as such it is thestrongest of the various components, and the one susceptibleof optimumefficiency of interaction with the beam. This circuit may becharacterized, in an alternative, as having a negative phase constantjust above the resonant frequency of the wires, a phase constant whichbecomes less negative as the frequency rises.

The invention has special application to backward wave oscillators. Insuch oscillators, an electron beam is projected past an interactioncircuit conducive to backward wave operation. Oscillations are set up bythe regenerative action of the electron beam which travels oppositely tothe'wave and transfers energy from regions of higher wave energy levelupstream along the circuit to regions of lower wave energy leveldownstream along the circuit. The terms upstream and downstream are usedhere and hereinafter to denote relative separation along the path ofelectron flow from the electron source. In the most common form ofbackward wave oscillator the oscillatory wave energy is abstracted bymeans of an output connection at the upstream end of the circuit wherethe oscillatory wave is a maximum while the downstream end of thecircuit is made substantially refiectionless to minimize the effect ofwave energy reflected from the output connection. In an alternativeform, the modulations set up on the electron beam by interaction withthe oscillatory backward traveling wave may be used to induce acorresponding wave in a suitable coupling element.

The invention will be more fully understood from the following moredetailed description taken in conjunction since the impedance of thecircuit is higher for such'a fundamental component, with a consequentimprovement with the accompanying drawings in which:

- Fig. 1A shows an axial longitudinal section of an 'arrangementincluding a slow wave circuit in accordance with the invention and arectangular wave guide coupling connection at one end thereof, Figs. 18through 1F are sections taken along lines BB, CC, DD, EE, and- FF,respectively, of the arrangement shown in Fig. 1A; Figs. 2 and 3 eachillustrate backward wave type oscillators wherein there is incorporateda slow wave cirquit of thekind shownv in Fig. 1; and

Figs. 4 and 5 are views taken in transverse cross section of alternativeforms of slow wave circuits in accordance with the invention.

With reference now more particularly to the drawings, the interactioncircuit arrangement shown in Figs. 1A through 1F comprises a hollow waveguide 10, advantageously of copper, which initially is of rectangularcross section and of standard dimensions for the operating band ofwave-lengths. As shown, the vertical dimension of the wave guidingpassage is gradually decreased with travel from left to right, by thetapered conductive inserts 11 and 11A which extend longitudinally alongthe guide and serve as a mode converter for transforming the transverseelectric mode characteristic of propagation in the rectangular waveguide to one suitable for propagation by the slow wave circuit. Theinserts 11 and 11A are positioned in mirror symmetry relative to anaxial plane parallel to the broad walls of the guide. In Fig. IE, it isseen that each of the inserts extends across the full width of the waveguiding passage. At, a point along the wave guide at which the verticaldimension of the wave guiding passage has been appreciably reduced, eachof a plurality of wirelike conductive elements 12, advantageously ofribbon type, stretches across the width of the wave. guiding passageforming a linear array extending axially along the wave guide in theplane of sym metry of the two insert members. This linear arrayadvantageously begins at the point where the vertical dimension of thewave guiding passage is least. Beyond the point where the linear arrayof Wires begins, each of the insert members is grooved. The two insertsare advantageously grooved in a manner which continues to make the planeof the wires a plane of mirror symmetry. It will be convenient todescribe in detail only the groove in insert member 11. Its groove isstepped, having a central portion of uniform width equal advantageouslyto approximately one half the length of the wires along the uniform mainportion of the array and two symmetrically stepped end portions, whichmay be viewed as ledges 13 and 14 in the side walls. Along a transitionregion the depth of the central portion 15 of the grooving increasesgradually with axial distance, thereby increasing gradually theseparation of the central portion of the wires from the bottom surfaceof the wave guide. Over this transition region the vertical separationof the stepped portions 13, 14 from the ends of the wire elementsremains substantially uniform but the width of such portions decreasesgradually in a manner etfectivc- 1y to shorten the free lengths of thewire elements. Inthe main portion beyond the transition region, thegeometry of the circuit remains uniform. In a similar manner the insert11A is grooved to form a central groove 15A be tween stepped portions13A and MA. A comparison of Figs. 13 through 1F illustrates the mannerof transition from a hollow rectangular wave guide to a slow wavecircuit in accordance with the invention. The overall length of thetransition region advantageously is at least several operatingwavelengths long to minimize reflections in the course of the modeconversion.

The arrangement described is adapted either for transferring wave energysupplied to the rectangular wave guide to the slow wave circuit or fortransferring wave energy from the slow wave circuit to the rectangularwave guide. Of course, the slow wave circuit can be provided with. arectangular wave guide connection at each end in an analogous manner. Asshown in Fig. 1F, which is a transverse section of the fully developedform of the slow wave circuit, each' of the wire elements 12 is loadedat its ends withrespect to the center portion by ledges 13A and 14Asymmetric with respect to ledges 13 and 14, respectively. From thestandpoint of the entire length of each wire, the cen' tral grooves 15and 15A may be viewed as reducing the capacitance of the central portionrelative tothe two end ortions. Alternatively, fr'onithe standpoint ofonly thecentral portion of the length of each wire, the portionsextending beyond into the side ledges may be viewed as the addition ofinductive loading. Viewed in either fashion, the net efiect is to reducethe cutoff wavelength of the wave guide to a value less than theresonant wavelength of the entire length of the wires. It is lessimportant that the depths of central grooves 15 and 15A be similarinasmuch as such dimensions are relatively immaterial provided they aresuflicient. How: ever, to preserve the symmetry, such dimensions arehere shown equal. The interior of the wave guiding passage of this kindwill for purposes of brevity be characterized as cruciform in crosssection. A circuit of this kind would be coupled to a rectangular waveguide by a transition region which includes a pair of symmetricallypositioned mode conversion members of the kind described above. It maybe desirable, for special applications, to have'the loading asymmetric,in which case the spacing of ledges 13A and 14A from the wire elementsmay be different from that of ledges 13 and 14 and/or the ledges may beof different widths.

In the backward wave oscillator shown in Fig. 2, the slow wave circuit20 is of the kind shown in Figs. 1A through 1F. As discussed above, itincludes a plurality of wire elements 12 extending transversely across awave guide 10 having a wave passage therethrough which is substantiallycruciform in cross section. The wires form a linear array which extendsalong the axis of the guide. The slow wave circuitis coupled to asection of hollow wave guide of rectangular cross section by way of atransition region. which is formed by a pair of insert members 11, 11Afor effecting the necessary mode conversion.

For providing an electron beam for flow past the wave circuit, anelectron gun, shown schematically simply as the cathode 32, ispositioned beyond one end of the circuit within an evacuated enclosurerepresented by the broken line 33. The beamis projected past the circuitunder the impetus of accelerating forces provided by a potentialdifference applied between the cathode and the slow wave circuit. Asuitable aperture is provided in the wall of the wave guide 10 forentrance of the beam into the wave guiding passage. The electronsadvantageously flood. the region surrounding the linear array of wires.To minimize transverse excursions of electrons as they flow past thelinear array, the tube is advantageously completely immersed in astrong, axial magnetic field. Such a field may be established either bya permanent magnet or anelectromagnet. In the interest of simplicity, ashowing of such flux producing equipment has been omitted from thedrawing. In a tube of this kind, it is unnecessary to provide a separatecollector electrode at the end of the slow wave circuit but instead thebeam is there permitted to diverge by the insertion of permeablematerial which results in a distortion therealong of the focusing fieldand so is collected by the Walls of the wave guide. i

In this embodiment, asin the usual form of backward wave oscillator, theoscillatory wave is abstracted at the upstream end of the slow wavecircuit. To this end, the upstream end of the slow wave circuit iscoupled to a rectangular wave guide section 34 which serves as theoutput coupling connection. It facilitates immersion in an axialmagnetic field if 'transverse dimensions of the coupling connection areminimized and so the rectangular wave guide section-isprovided with apair of right angle bends. A pressure tight glass window 35 is providedat the end of this section by way of which output energy may be suppliedto a rectangular wave guide connection.

-In an oscillator of this kind, it is usually unnecessary to take anyspecial precautions to make the downstream end of the slow wave circuitreflectionless. It is sufficient merely to extend thelength of theslowwave circuit beyond the point at which the electron stream is firstallowed to diverge so that the loss' inherent in the 'slow: wave circuitprovides the necessary absorption of any incident wave energy.

The operation is of the kind familiar to a backward wave oscillator.Noise components in the electron stream set up noise waves whichpropagate upstream in the slow wave circuit. By appropriate choice ofthe velocity of the beam, interaction in a backward wave mode is badwith the one of such waves of a desired frequency, and

oscillations result at such frequency when the beam current and thecoupling elficiency are sufficiently high. Ac-

cordingly, the frequency of oscillations is tuned by changes in thevelocity of the beam which, in turn, is controlled by the acceleratingvoltage applied between the cathode and circuit. By modulating thisvoltage with signal information, a frequency modulated output wave canbe achieved. In particular, the interaction is most efficient in anoperating band of frequencies at which the wavelength is slightlyshorter than twice the length of the wire elements along the smoothportion of the slow wave circuit.

Fig. 3 shows schematically an alternative form of back-' ward waveoscillator 40. Herein an interaction circuit 20 of the kind previouslydescribed is employed pri marily to modulate an electron beam inacordance with an oscillatory wave. Thereafter such beam modulation ismade to induce an oscillatory wave into a slow wave circuit 41positioned downstream with respect to the backward wave circuit. Theslow wave circuit 41 is one especially adapted for operation in aforward wave mode. Since a wide variety of forms of such a forward wavecircuit is known, it-is here shown schematically. Typically, such aforward wave circuit may be a wire or ribbon helix. For operation atmillimeter'wavelengths, the forward wave circuit may advantageously beof the form described in the aforementioned Karp-Yocom application,comprising a linear array of wire elements positioned in a wave guidewhich is provided with a central ridge. The general principles andadvantages of a backward wave oscillator of this kind are described in acopending application Serial No. 392,946 filed November 18, 1953, by H.Heffner and R. Kompfner.

Within an evacuated enclosure shown as the broken line 42, at oppositeends an electron gun 43 and a collector electrode 44 serve to define apath of flow of an electron beam. Suitable equipment (not shown) is usedto minimize transverse components of flow. Positioned upstream along thepath of fiow is a slow wave interaction circuit of the kind shown inFigs. 1A through 1F which provides backward wave type interaction withthe electron flow. The characteristics of this circuit are made to besuch that a backward traveling oscillatory wave is induced in thiscircuit in the manner characteristic of the oscillator described above.In this instance, the slow wave circuit 20 is terminated in itscharacteristic impedance at both ends so that the oscillatory wave setup therein is absorbed with minimum reflection. However, concomitantwith the growth of this oscillatory wave in the slow wave circuit, thereresults a corresponding modulation of the electron beam. Such beammodulation may be viewed as the growth of space charge waves on theelectron beam. Such space charge waves grow with progress in thedirection of electron flow past the slow wave circuit 20 andsubsequently induce a forward traveling wave in the slow Wave circuit 41of a frequency corresponding to the wave set up initially in thebackward wave circuit 20. The circuit 41 is designed so that the forwardinduced wave interacts with the electron beam in the mannercharacteristic of forward wave type traveling wave amplifier. Theamplified wave is abstracted at the downstream end of circuit 41. Theupstream end of circuit 41 preferably is terminated. Alternatively, thecircuit 41 may be designed to have nonreciprocal attenuationcharacteristics, low in the forward direction, high in the backwarddirection. It is important that the cir- 6 cuit 41.'-be designedjnot beoscillate independently in a backward wave mode if the full advantagesof this form of oscillator are to be realized.

The principles of the invention may be embodied in alternative forms ofslow wave circuits. Various expedients are possible for arranging thatin a linear array of wire-like elements extending along a wave guide,each of the elements be capacitively loaded less at its ends than in thecenter whereby the length of each element becomes longer than half thecutoff wavelength of the guide.

In the circuit 50 shown in Fig. 4, a plurality of curved wire elements51 extend in a linear array between the narrow side walls of arectangular wave guide 52.

In. the circuit shown in Fig. 5, a plurality of wire elements 61 extendin a linear array between opposite vertices of a rectangular wave guide62. i

It isto be understood that the various embodiments described are merelyillustrative of the general principles of the invention. Variousarrangements may be devised by workers skilled in the art withoutdeparting from the spirit and scope of the invention. For example, theslow wave circuits which form the principal feature of the invention maybe provided with coupling connections at their two ends and utilized intraveling wave amplifiers of the backward wave type.

' What is' claimed is:

- 1. In a backward wave oscillator, means for forming an electron beam,an interaction circuit positioned along the path of flow of said beamcomprising a hollow wave guide substantially cruciform in cross sectionin which between a pair of surfaces extends transversely a plurality ofaxially spaced wirelike conductive elements in a linear array, and acoupling connection to the upstream end of said interaction circuitcomprising a hollow wave guide of rectangular cross section coupled tosaid interaction circuit by way of a mode converting member.

2. In a backward wave oscillator, means for forming an electron beam, abackward wave mode interaction circuit positioned upstream along thepath of flow comprising a hollow wave guide substantially cruciform incross section in which between a pair of surfaces extends transversely aplurality of wirelike conductive elements.

3. In a backward wave oscillator, an interaction circuit comprising ahollow conductive wave guide having a wave path therethrough whose crosssection is substantially cruciform, and a plurality of wirelike elementsextending between an opposite pair of surfaces of said wave guide in alinear array parallel to the axis of the wave guide, the successiveelements of the array extending parallel to each other and being spacedapart in the direction of wave propagation along said wave guide.

4. In a device which utilizes the interaction between an electron beamand a negative spatial component of an electromagnetic wave foramplifying the wave, means for forming an electron beam and forprojecting said beam along a predetermined path, and a slow wave circuitalong said path for propagating an electromagnetic wave in couplingrelation with said electron beam, said slow wave circuit comprising ahollow conductive wave guide positioned to surround the beam path alonga substantial portion of the length of the path and characterized inthat the inner surface of the guide defines a wave path which in crosssection is substantially cruciform in shape, a plurality of wirelikeelements extending from opposite points of the inner surface of the waveguide to form a linear array along the beam path, successive elements ofthe array extending parallel to each other and being spaced apart in thedirection of the beam path.

5. In a device which utilizes the interaction between 7 a charged(particle beam and an electromagnetic wave for amplifying the wave,means for forming'a beam of charged particles and for projecting saidbeam along a predetermined path, andia slow wave circuitfor propagatingan electromagnetic wave in coupling relation-withtsaid beam, said slowwave circuit comprising a conductively bounded wave guiding path and asuccession of wirelike elements extending across said path, successiveelements of the succession extending parallel to each other and beingspaced apart in the direction of wave propagation along said path, saidslow rwave-circuit-rbeing characterized in that the cross sectionof itsconductively bounded wave guiding path includes a first region having.a. predetermined transverse dimension and a second region having alarger transverse dimension measured. in the same direction as saidpredetermined transverse dimension and further characterized in that thesuccession of wirelike elements extends across said wave path 'intheregion ofits larger transverse dimension.

6. In a device which utilizes the interaction. between a chargedparticle beam and. an electromagnetic wave for amplifying the wave,means for forminga beam of charged particles and for projecting saidbeam along a predetermined path, and a slow wave circuit for propagatingan electromagnetic wave in coupling relation with said beam, said slowwave circuit comprising a conductively bounded wave guiding path. and anarray of wirelike elements extending across said path, successiveelements of the array extending parallel to each other andbeing spacedapart in thedirection of wave propagation along said path, said slowwave circuit being characterized in that the cross section of itsconductively bounded wave guiding path includes a first region having apredetermined transverse dimension and a second region having atransverse dimension' measured in'the same direction as said predeter-'mined transverse dimension equal to approximately twice the transversedimension of the first region and further characterized in thatthefarray of wireiike elements extends across said wave path along saidsecond region.

7. In a device which utilizes'tlie interaction b'etwcen an electron beamand a spatial component of an electromagnetic wave for amplifying thewave, means for forming an electron beam and for projecting-said beamalong a predetermined-path, aslowwave circuit'along said path forpropagating an electromagnetic wave in coupling relation with saidelectron beam, and" coupling means connected at one'end of said? slowwave circuit for extracting wave energy therefrom, said slow wavecircuit comprising a hollow conductive waveguide positioned to surroundthe beam path along a substantial portion of the length of the path andcharacterized inthat the inner surface of the guide defines a wave pathwhich in cross? section is substantially cruciform in shape, a pluralityof wirelike elementsex'tending from opposite points of the inner surfaceof the wave guide to form a linear array along the beam path, successiveelements of the array extending parallel toeach other and being? spacedapart inthe direction of the beam' path.

References Cited in the file of thispatent UNITED STATES PATENTS

